linux-kernel - Re: [RFC PATCH 08/15] smp: Don't warn if irq are disabled but we don't wait for the ipi

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Message-ID: <20101221000201.GF1715@nowhere>
Date:	Tue, 21 Dec 2010 01:02:03 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 08/15] smp: Don't warn if irq are disabled but we
 don't wait for the ipi

On Mon, Dec 20, 2010 at 05:03:59PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> > The comment in smp_call_function_single() says it wants the irqs
> > to be enabled otherwise it may deadlock.
> > 
> > I can't find the reason for that though, except if we had to wait
> > for a self triggered IPI but we execute the local IPI by just
> > calling the function in place.
> > 
> > In doubt, only suppress the warning if we are not waiting for the
> > IPI to complete as it should really not raise any deadlock.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Lai Jiangshan <laijs@...fujitsu.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Anton Blanchard <anton@....ibm.com>
> > Cc: Tim Pepper <lnxninja@...ux.vnet.ibm.com>
> > ---
> >  kernel/smp.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 12ed8b0..886a406 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -289,7 +289,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
> >  	 * send smp call function interrupt to this cpu and as such deadlocks
> >  	 * can't happen.
> >  	 */
> > -	WARN_ON_ONCE(cpu_online(this_cpu) && irqs_disabled()
> > +	WARN_ON_ONCE(cpu_online(this_cpu) && wait && irqs_disabled()
> >  		     && !oops_in_progress);
> >  
> >  	if (cpu == this_cpu) {
> 
> You just deadlocked the machine.. note how you can still wait on the
> previous csd in csd_lock().

Ah right.

I should then use __smp_call_function_single().
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